This invention relates to the field of medical electrical lead bodies for use with various implantable electronic sensing and stimulation devices such as cardiac pacemakers, implantable cardioverter defibrillators and neurostimulators, and to the method of making such implantable lead bodies.
Implantable medical electrical stimulation and/or sensing leads are well known in the fields of cardiology and neurology. Stimulation leads transmit therapeutic energy from an electrical pulse generator to the respective tissue or nerve. Sensing leads transmit electrical signals from tissue to a remote sensor. Common applications in cardiology include the treatment of various arrhythmia, (e.g. bradycardia, and tachycardia). Applications in neurology include the treatment Parkinson's Disease, epilepsy, and chronic back pain. All such medical electrical leads are herein referred to as “Implantable Leads.”
Implantable leads must have excellent mechanical integrity, electrical isolation between circuits, biocompatibility, and must be flexible enough to accommodate physiologic geometry. Implantable leads must also be durable enough to accommodate the repeated flexure due to attachment and dynamic affects of anatomical features, e.g. a beating heart, a spinal cord, neck, a peripheral nerve, etc.
Known leads for use with implantable electrical stimulation devices such as cardiac pacemakers, implanted defibrillators, and neurostimulation devices are typically constructed of a lead body having an electrode assembly at the distal end, and a connector assembly at the proximal end of the lead body to connect to a pulse generator.
A lead body consists of at least one insulated electrical conductor and an outer insulation layer of tubular form coaxially surrounding the electrical conductor. Current lead body constructions for cardiac and neurological applications generally fall into two categories, coaxial and multilumen designs. A coaxial lead body typically consists of one or more helically wound coils, concentric to one another. Each coil is separated by a tubular form of insulation.
Multilumen constructions typically consist of a silicone extrusion with a desired cross section to house a combination of helically wound coils and conductors. In either coaxial or multilumen construction, a fluoropolymer material, such as Ethylene Tetrafluoroethylene (ETFE) is applied to the conductor materials. This material acts as a chemical barrier to help prevent metal ion oxidation—a reaction of the metal conductors which occurs from the release of hydrogen peroxide from macrophages.
The implantable leads described above have several disadvantages. Due to the softness of silicone, lead bodies made from that material are prone to damage during implantation and often fail (in-vivo) mechanically due to tearing, abrasion, and depression. Depression is a compressive force applied to the lead which causes the material to fracture. Silicone leads may also result in cases of acute allergic responses in some patients.
Polyurethane materials are frequently used as an alternative to silicone for added mechanical strength and lower coefficient of friction. Polyurethanes have been used in direct replacement of silicone and/or as an outer covering, or sheath for leads. Polyurethane materials and the respective leads have been known to fail due to environmental stress cracking resulting from metal ion oxidation which ultimately leads to material delamination. Such failures are known to result in pieces of insulation being released into the blood stream creating a high risk of adverse affects, including ischemic stroke.
Implantable lead wires using insulation materials other than the conventional silicones and polyurethanes have also been suggested. U.S. Pat. No. 4,573,480 describes an implantable electrode lead body in the form of a helically wound conductor having a tubular insulating layer surrounding the wire in which the tubular insulating layer is porous polytetrafluoroethylene (herein after PTFE) having a pore size limited to a maximum size described as “being essentially impervious to body fluids to prevent tissue ingrowth.” This patent also teaches that the tubular porous PTFE insulating layer may alternatively be provided with an outer covering of smooth impervious material.
As the design of implantable electrical leads has progressed, there has been a general trend toward reduction in the diameter of the lead body, with further reduction desired. A lead of small body diameter may reduce the risk of internal trauma and infection, permit improved navigation through potentially tortuous geometry and simplify placement in small anatomical features. However, maintaining adequate mechanical integrity, biocompatibility, and electrical performance, which remain critical for patient safety and device effectiveness, are increasingly difficult with reductions in diameter.